Variable-rate defoliant promising

The variable-rate application of defoliants using remote sensing technology could save Mid-South cotton producers $4 to $6 per acre in chemical costs, according to a three-year LSU AgCenter study.

The study was part of an overall project for adapting precision agricultural techniques for Mid-South cotton production. LSU AgCenter Entomologist Ralph Bagwell discussed the study at InfoAg Midsouth 2005, in Tunica, Miss.

The study compared yields, grades and costs of variable-rate applications to blanket applications made by air on large, farm fields. The applications were made in long strips of 8 acres to 15 acres each.

Variable-rate prescriptions were based on remote sensing imagery and used a normalized vegetative difference index of the field. The NDVI “shows the relative difference in biomass across the field,” noted Bagwell.

“The first prescriptions (2002) that we wrote were extremely coarse,” Bagwell said. “We were trying to be conservative because no one had written a variable-rate prescription for a defoliant before.”

In the first year, areas of the field where blanket applications were made received 10 ounces of a defoliant per acre. Compared to variable-rate applications in the field, “we saved about $1.50 per acre on average.”

The study did not include the costs of making the application.

The first year of the study did not show a significant difference in the cotton loan rate between the two application methods, “indicating that there was not a significant influence from the defoliant on lint values. Lint yields were also the same.”

The study took on a little more complexity in year two. The cooperating grower or his consultant determined the broadcast application rate and the structure for the variable-rate application was based on an NDVI image.

The second year included a two-step defoliation program on a 230-acre field with extremely high variability. Percent open ranged from 95 percent to 50 percent.

The first broadcast application was 8 ounces of a defoliant. The variable-rate application ranged from 8 ounces to 4.8 ounces.

Overall cost for the first broadcast application was around $12 per acre compared to $9.50 for the variable-rate application.

The second broadcast application was $11.25, compared to the variable-rate application of $6.19. The biggest difference in cost in the second application was that the grower was about to go to a zero rate of boll opener on parts of the field, while the broadcast application remained at the same rate across the field.

That year provided the biggest overall difference of $7.74 between variable-rate and broadcast applications, according to Bagwell. “This figures out to about a 33 percent savings, which is the biggest savings we have observed. We did not see a significant difference in lint yield or overall grade or quality between any of the treatments.”

In 2004, the study expanded again and used a two-step program. Across all locations, the broadcast application was about $13, compared to a variable-rate cost of about $10.50. On the second application, broadcast costs averaged $14, versus a variable-rate cost of about $12. The overall savings was about $4.75, roughly a 21 percent savings.

Bagwell tells growers that they can expect a 20 percent savings in variable-rate defoliation cost versus broadcast, with a range of 10 percent to 33 percent.

“If you have an applicator, making a variable-rate application of defoliant seems to be profitable. But by all means keep it as simple as possible when you set up rate structures for variable-rate applications. Most systems have a difficult time making multiple changes very quickly.”

Plant growth regulators did not produce the level of savings that defoliants did, according to Bagwell. “We had an average broadcast application rate of plant growth regulators of 12.8 ounces per acre, compared to an average variable-rate application rate of 7.1 ounces, with an average savings of $1.56 an acre. That's just not a lot of money with the cost of plant growth regulators today.

“In-Time charges about $1.50 per image and an aerial applicator would charge an extra dollar per acre to apply the product variable-rate. So it would cost $2.50 to make the application. According to this study, variable-rate application of plant growth regulator is going to have to have some added benefit.” Those savings could come in increased yield over a broadcast application, according to Bagwell.